The easiest way to record whether or not you have seen a film on your Watchlist is to rate the title. That said, you can sort your Watchlist and your list of Ratings by the date they were added to your list or the Release Date of the film.

Simply navigate to the list you want to sort and choose the three horizontal bars in the upper left:

Then choose the dropdown called 'Sort By':

This will allow you to select from the sort options, such as Date Added or Release Date: